The combustion of a carbonaceous material such as a solid carbonaceous fuel by reaction with a source of gaseous oxygen is well known. In such a reaction, an amount of air or oxygen equal to or greater than that required for complete combustion is used, whereby the gaseous effluent contains carbon dioxide with little, if any, carbon monoxide. It is known also to carry out the gasification or partial oxidation of solid carbonaceous materials or fuels employing a limited quantity of oxygen or air so as to produce primarily carbon monoxide and hydrogen.
The gases resulting from partial combustion or gasification of solid carbon-containing fuels such as coal have value as residential and industrial fuels. They also have value as starting materials for synthesis of chemicals and fuels and as an energy source for generation of electricity. These uses have long been recognized and partial gasification has been practiced on varying scales throughout the world. In the case of coal gasification, a number of different gasification processes have been developed to take into account factors such as the coal source employed, the gasifying medium used and the use sought to be made of the product gas.
While these processes may be classified in a variety of ways, they generally fall into two distinct groups. The two groups are distinguished with respect to the condition in which the non-carbonaceous, mineral residue is removed from the gasification zone. The residue is removed as a dry ash in a nonslagging operation or as a slag in a slagging operation. These two processes are different primarily due to the temperatures employed in the gasification zone itself. The nonslagging gasifiers are operated at lower reaction temperatures, usually less than 1400.degree. C. This is below the temperature at which the contained ash will fuse. The temperatures employed in slagging gasifiers are usually 1500.degree.-2700.degree. C. These temperature are sufficient to convert the dry ash into a molten slag.
Advantages exist for gasification processes in each of the two process groups. The processes employing slagging coal gasifiers, however, are generally considered to be the most flexible at least in terms of the variety of coal feedstocks which can be employed. The operation of coal gasifiers under nonslagging conditions is generally limited to coals of low ash content because of the difficulty in removing ash with grates and other mechanical devices. In contrast, in operation at slagging conditions almost any coal can be suitably employed because the ash becomes a free-flowing fluid under slagging conditions. As a result, the fluid ash is quite simply and easily removed from the gasifier. A good general review of a variety of coal gasification processes appears in the Kirk-Othmer Encyclopedia of Chemical Technology, 2nd Ed., Vol. 10, pp. 353-388, Interscience (1966).
One process employing a slagging coal gasifier which has had rather wide application is the Koppers-Totzek process. This process which is described in an article by F. Totzek in "Brennstoff-Chemie," Vol. 34, pp. 361-367 (1953), has the capability of handling just about any coal including lignites with up to 30% ash or mineral contents. A significant portion of the molten slag is removed at the bottom of the gasifier. However, the product gas of this process, like other processes employing slagging gasifiers, still contains a significant quantity of mineral matter. The mineral matter is in the form of a suspension or mist of molten or partly molten particles. In this molten state the ash is sticky.
The slag in coals is caused primarily because of the impure nature of coal and the mineral matter in typical coals. These impurities include mixtures of silica and various metal oxides. The molten or partly molten slag will not have a specific melting point. Instead it will solidify over a wide melting range which may cover many hundreds of degrees which can make processing difficult.
It is typically desired to cool the coal gasifier effluent prior to further processing. In a typical application, the gas leaving the reactor has a temperature, as a rule, higher than 1400.degree. C. (2552.degree. F.), preferably from about 2650.degree. F. to about 3000.degree. F., at which the ash is quite fluid. For further processing, this crude product gas is typically cooled down to, for example 300.degree. C. (572.degree. F.). In the cooling the gas will pass through a broad range of temperatures at which the slag is sticky. The slag from coal usually is sticky at temperatures greater than about 1700.degree. F. (927.degree. C.). Once the slag particles are no longer sticky, they can be easily removed by known techniques such as cyclones, bind separators, filters or similar devices.
In the transition between being highly fluid molten liquid and solid nonsticky particles, these slag particles exhibit sufficient stickiness that they can cause extreme difficulties in processing. That is, the slag adheres to and forms deposits on walls, valves, outlets, etc., of process equipment immediately downstream of the gasifier. These deposits tend to build up and as a result interfere with good operation of the process and even lead to complete blocking.
A previous process has added coal directly to the cooling gas (see U.S. Pat. No. 4,054,424). Such a process is disadvantageous, however, since the gas mixture is cooled too much to provide complete reaction of the added coal particles. As a result, undesirable hydrocarbons may be formed, such as ringed compounds, that will have to be removed thus requiring additional processing.
Another previous process, disclosed in U.S. Pat. No. 2,987,387, provides for diverting of vaporized slurry feed water (i.e., steam) in excess of that required for gasification temperature control. The excess is diverted for use in quenching the gasifier effluent. The diverted excess contains only trace amounts of reactive materials such as powdered or volatilized coal. The weight ratio of steam to coal is less than 1 to 0.1. The excess steam reacts with unreacted coal in the gasifier effluent as well as the trace coal in the excess steam. The addition of steam can cause plugging and fouling problems since it is a condensible substance. In a dry feed system, the addition of steam can wet the coal particles, forming a "mud." Any condensed steam will also have to be separated. Thus another separations unit is required.
Accordingly, it would be advantageous to have a process for cooling the product gas of a slagging coal gasifier in which the harmful effects of the stickiness of molten slag particles contained therein is minimized or even completely eliminated and the disadvantages of previously known processes are avoided.